Many processes are known in the art for removing chromium as a chromate or dichromate ion from water. Most of these processes use well known anion exchange resins due to the chromate and dichromate existing in the anion form. For example, in U.S. Pat. No. 3,664,950 an improved anion exchange process for removal of chromates from water is disclosed. The process utilizes a bed of basic anion exchange resin and the combination of upflow exhaustion of the chromates from the water and downflow regeneration of the resin. It is noted that unlike the absorbent utilized in the process of the instant invention, this strongly basic anion exchange resin which is disclosed in this reference, is clearly an organic material; such as the resins described at column 3, lines 20-23 of the patent. Furthermore, the patentee did not address himself to the problem of removing chromate or dichromate from the material which comprises a substantial amount of chlorate as a possible competing anion for the anion exchange resin.
The patentee of U.S. Pat. No. 3,835,001 teaches a process for adsorption of dichromate ions in the presence of chloride and chlorate ions. Again an organic resin (a strongly basic quaternary resin) is used as the adsorbent.
Various other patents disclose processes for removing chromium from aqueous solutions by means of a combination of anion and cation exchange. For example, see U.S. Pat. Nos. 3,885,018 and 3.903,237 which disclose a process wherein an aqueous hexavalent chromium-containing solution (in relatively low concentration) is concentrated by treatment first through a first cation exchange zone followed by treatment in an anion exchange zone. Again, the anion exchange resin which is utilized to separate the hexavalent chromium in its chromate anion form is exemplified by an organic material, i.e. a quaternary ammonium ion-containing material.
Other U.S. Patents which disclose the removal of chromate or the recovery of chromic acid from aqueous solutions include U.S. Pat. Nos. 4,049,772 and 4,145,281. Note in both of these patents the anion exchange material which is utilized to remove the chromic acid or chromate is disclosed to be an organic resin type. Note both of these patents are concerned with removal of chromate or chromic acid from plating bath and cooling tower blowdown water. In neither case would large amounts of chlorates be found in combination with the chromate, chromic acid containing solution.
An alternative process for removing chromium from polluted waters (which as taught, may originate from the use of chromium in plating baths) is by means of reacting soluble chromium with a sulfide in the presence of a water-soluble ferric salt to form a chromic sulfide precipitate.
In U.S. Pat. Nos. 3,332,737 and 3,382,034, it is taught that chromate may be removed from a solution by adsorption on a Group VI hydrous oxide ion exchanger. Although ferric oxide is mentioned in both of these patents as an adsorbent, there is no teaching that ferric oxide may be used to remove chromium as shown in the instant process.
In one article, James S. Davis, James O. Leckie, Journal of Colloid and Interface Science, Vol. 74, March, 1980, p. 32, it is disclosed that an adsorbent comprising Fe(O)OH may be used to adsorb chromates. However, the actual solutions tested contained only about 25 micrograms of chromium, an amount not sufficient to predict the efficacy of a process for the removal of the larger amounts of chromium found in solutions which are found in commerce (e.g. plating wastes, effluent from the electrolytic manufacture of chlorate from chloride, etc.) Moreover, as disclosed in another paper by the same authors, James A. Davis, James O. Leckie, Journal of Colloid and Interface Science, Vol. 67, October, 1978, p. 90, the adsorbent is an FeO(OH) gel, not the granular material utilized in the instant process.
The only commercially practiced process for chromium removal from chlorate-containing solutions is to precipitate the chromium as barium chromate and remove it by filtration. The barium process, however, has some undesirable characteristics. The chemical cost is high, caused in part by sulfates in the chlorate product. The chromium values are not recoverable for reuse. Also, in order to reduce the final chromium content below about 50 ppm, a substantial excess of barium must be added. The desired level of Cr is less than 10 ppm in the final product. Finally, disposal of the barium chromate sludge adds an additional cost.